Take the number of the street address, drop the last digit, divide by two (essentially dividing the address by 20 but accounting for odd numbers), and add or subtract the number in the chart. For example, 945 Madison Avenue (Whitney Museum of American Art, by the way) would yield the following equation:

945 / 2 = 47 + 27 (from the chart below) = 74th Street.*The entrance is actually closer to 75th Street, but the museum takes up the whole block so it is just as close to 74th Street.